tacred.ml

(************************************************************************)
(*  v      *   The Coq Proof Assistant  /  The Coq Development Team     *)
(* <O___,, *   INRIA - CNRS - LIX - LRI - PPS - Copyright 1999-2017     *)
(*   \VV/  **************************************************************)
(*    //   *      This file is distributed under the terms of the       *)
(*         *       GNU Lesser General Public License Version 2.1        *)
(************************************************************************)

open Pp
open CErrors
open Util
open Names
open Term
open Libnames
open Globnames
open Termops
open Environ
open EConstr
open Vars
open Find_subterm
open Namegen
open CClosure
open Reductionops
open Cbv
open Patternops
open Locus

module RelDecl = Context.Rel.Declaration
module NamedDecl = Context.Named.Declaration

(* Errors *)

type reduction_tactic_error =
    InvalidAbstraction of env * Evd.evar_map * EConstr.constr * (env * Type_errors.type_error)

exception ReductionTacticError of reduction_tactic_error

(* Evaluable reference *)

exception Elimconst
exception Redelimination

let error_not_evaluable r =
  user_err ~hdr:"error_not_evaluable"
    (str "Cannot coerce" ++ spc () ++ Nametab.pr_global_env Id.Set.empty r ++
     spc () ++ str "to an evaluable reference.")

let is_evaluable_const env cst =
  is_transparent env (ConstKey cst) && 
    (evaluable_constant cst env || is_projection cst env)

let is_evaluable_var env id =
  is_transparent env (VarKey id) && evaluable_named id env

let is_evaluable env = function
  | EvalConstRef cst -> is_evaluable_const env cst
  | EvalVarRef id -> is_evaluable_var env id

let value_of_evaluable_ref env evref u =
  match evref with
  | EvalConstRef con -> 
    let u = Unsafe.to_instance u in
    EConstr.of_constr (try constant_value_in env (con,u)
    with NotEvaluableConst IsProj -> 
      raise (Invalid_argument "value_of_evaluable_ref"))
  | EvalVarRef id -> env |> lookup_named id |> NamedDecl.get_value |> Option.get

let evaluable_of_global_reference env = function
  | ConstRef cst when is_evaluable_const env cst -> EvalConstRef cst
  | VarRef id when is_evaluable_var env id -> EvalVarRef id
  | r -> error_not_evaluable r

let global_of_evaluable_reference = function
  | EvalConstRef cst -> ConstRef cst
  | EvalVarRef id -> VarRef id

type evaluable_reference =
  | EvalConst of constant
  | EvalVar of Id.t
  | EvalRel of int
  | EvalEvar of EConstr.existential

let evaluable_reference_eq sigma r1 r2 = match r1, r2 with
| EvalConst c1, EvalConst c2 -> eq_constant c1 c2
| EvalVar id1, EvalVar id2 -> Id.equal id1 id2
| EvalRel i1, EvalRel i2 -> Int.equal i1 i2
| EvalEvar (e1, ctx1), EvalEvar (e2, ctx2) ->
  Evar.equal e1 e2 && Array.equal (EConstr.eq_constr sigma) ctx1 ctx2
| _ -> false

let mkEvalRef ref u =
  match ref with
  | EvalConst cst -> mkConstU (cst,u)
  | EvalVar id -> mkVar id
  | EvalRel n -> mkRel n
  | EvalEvar ev -> EConstr.mkEvar ev

let isEvalRef env sigma c = match EConstr.kind sigma c with
  | Const (sp,_) -> is_evaluable env (EvalConstRef sp)
  | Var id -> is_evaluable env (EvalVarRef id)
  | Rel _ | Evar _ -> true
  | _ -> false

let destEvalRefU sigma c = match EConstr.kind sigma c with
  | Const (cst,u) ->  EvalConst cst, u
  | Var id  -> (EvalVar id, EInstance.empty)
  | Rel n -> (EvalRel n, EInstance.empty)
  | Evar ev -> (EvalEvar ev, EInstance.empty)
  | _ -> anomaly (Pp.str "Not an unfoldable reference.")

let unsafe_reference_opt_value env sigma eval =
  match eval with
  | EvalConst cst ->
    (match (lookup_constant cst env).Declarations.const_body with 
    | Declarations.Def c -> Some (EConstr.of_constr (Mod_subst.force_constr c))
    | _ -> None)
  | EvalVar id ->
      env |> lookup_named id |> NamedDecl.get_value
  | EvalRel n ->
      env |> lookup_rel n |> RelDecl.get_value |> Option.map (lift n)
  | EvalEvar ev ->
    match EConstr.kind sigma (mkEvar ev) with
    | Evar _ -> None
    | c -> Some (EConstr.of_kind c)

let reference_opt_value env sigma eval u = 
  match eval with
  | EvalConst cst ->
    let u = EInstance.kind sigma u in
    Option.map EConstr.of_constr (constant_opt_value_in env (cst,u))
  | EvalVar id ->
      env |> lookup_named id |> NamedDecl.get_value
  | EvalRel n ->
      env |> lookup_rel n |> RelDecl.get_value |> Option.map (lift n)
  | EvalEvar ev ->
    match EConstr.kind sigma (mkEvar ev) with
    | Evar _ -> None
    | c -> Some (EConstr.of_kind c)

exception NotEvaluable
let reference_value env sigma c u =
  match reference_opt_value env sigma c u with
    | None -> raise NotEvaluable
    | Some d -> d

(************************************************************************)
(* Reduction of constants hiding a fixpoint (e.g. for "simpl" tactic).  *)
(* One reuses the name of the function after reduction of the fixpoint  *)

type constant_evaluation =
  | EliminationFix of int * int * (int * (int * constr) list * int)
  | EliminationMutualFix of
      int * evaluable_reference *
      ((int*evaluable_reference) option array *
       (int * (int * constr) list * int))
  | EliminationCases of int
  | EliminationProj of int
  | NotAnElimination

(* We use a cache registered as a global table *)

type frozen = constant_evaluation Cmap.t

let eval_table = Summary.ref (Cmap.empty : frozen) ~name:"evaluation"

(* [compute_consteval] determines whether c is an "elimination constant"

   either [yn:Tn]..[y1:T1](match yi with f1..fk end g1 ..gp)

   or     [yn:Tn]..[y1:T1](Fix(f|t) yi1..yip)
          with yi1..yip distinct variables among the yi, not occurring in t

   In the second case, [check_fix_reversibility [T1;...;Tn] args fix]
   checks that [args] is a subset of disjoint variables in y1..yn (a necessary
   condition for reversibility). It also returns the relevant
   information ([i1,Ti1;..;ip,Tip],n) in order to compute an
   equivalent of Fix(f|t) such that

   g := [xp:Tip']..[x1:Ti1'](f a1..an)
     == [xp:Tip']..[x1:Ti1'](Fix(f|t) yi1..yip)

   with a_k:=y_k if k<>i_j, a_k:=args_k otherwise, and
   Tij':=Tij[x1..xi(j-1) <- a1..ai(j-1)]

   Note that the types Tk, when no i_j=k, must not be dependent on
   the xp..x1.
*)

let check_fix_reversibility sigma labs args ((lv,i),(_,tys,bds)) =
  let n = List.length labs in
  let nargs = List.length args in
  if nargs > n then raise Elimconst;
  let nbfix = Array.length bds in
  let li =
    List.map
      (function d -> match EConstr.kind sigma d with
         | Rel k ->
             if
	       Array.for_all (Vars.noccurn sigma k) tys
	       && Array.for_all (Vars.noccurn sigma (k+nbfix)) bds
	       && k <= n
	     then
	       (k, List.nth labs (k-1))
	     else
	       raise Elimconst
         | _ ->
	     raise Elimconst) args
  in
  let reversible_rels = List.map fst li in
  if not (List.distinct_f Int.compare reversible_rels) then
    raise Elimconst;
  List.iteri (fun i t_i ->
    if not (Int.List.mem_assoc (i+1) li) then
      let fvs = List.map ((+) (i+1)) (Int.Set.elements (free_rels sigma t_i)) in
      match List.intersect Int.equal fvs reversible_rels with
      | [] -> ()
      | _ -> raise Elimconst)
    labs;
  let k = lv.(i) in
  if k < nargs then
(*  Such an optimisation would need eta-expansion
      let p = destRel (List.nth args k) in
      EliminationFix (n-p+1,(nbfix,li,n))
*)
    EliminationFix (n,nargs,(nbfix,li,n))
  else
    EliminationFix (n-nargs+k+1,nargs,(nbfix,li,n))

(* Heuristic to look if global names are associated to other
   components of a mutual fixpoint *)

let invert_name labs l na0 env sigma ref = function
  | Name id ->
      let minfxargs = List.length l in
      begin match na0 with
      | Name id' when Id.equal id' id ->
        Some (minfxargs,ref)
      | _ ->
	let refi = match ref with
	  | EvalRel _ | EvalEvar _ -> None
	  | EvalVar id' -> Some (EvalVar id)
	  | EvalConst kn ->
	      Some (EvalConst (con_with_label kn (Label.of_id id))) in
	match refi with
	  | None -> None
	  | Some ref ->
	      try match unsafe_reference_opt_value env sigma ref with
		| None -> None
		| Some c ->
		    let labs',ccl = decompose_lam sigma c in
		    let _, l' = whd_betalet_stack sigma ccl in
		    let labs' = List.map snd labs' in
		    (** ppedrot: there used to be generic equality on terms here *)
                    let eq_constr c1 c2 = EConstr.eq_constr sigma c1 c2 in
		    if List.equal eq_constr labs' labs &&
                       List.equal eq_constr l l' then Some (minfxargs,ref)
                    else None
	      with Not_found (* Undefined ref *) -> None
      end
  | Anonymous -> None (* Actually, should not occur *)

(* [compute_consteval_direct] expand all constant in a whole, but
   [compute_consteval_mutual_fix] only one by one, until finding the
   last one before the Fix if the latter is mutually defined *)

let compute_consteval_direct env sigma ref =
  let rec srec env n labs onlyproj c =
    let c',l = whd_betadeltazeta_stack env sigma c in
    match EConstr.kind sigma c' with
      | Lambda (id,t,g) when List.is_empty l && not onlyproj ->
          let open Context.Rel.Declaration in
	  srec (push_rel (LocalAssum (id,t)) env) (n+1) (t::labs) onlyproj g
      | Fix fix when not onlyproj ->
	  (try check_fix_reversibility sigma labs l fix
	  with Elimconst -> NotAnElimination)
      | Case (_,_,d,_) when isRel sigma d && not onlyproj -> EliminationCases n
      | Case (_,_,d,_) -> srec env n labs true d
      | Proj (p, d) when isRel sigma d -> EliminationProj n
      | _ -> NotAnElimination
  in
  match unsafe_reference_opt_value env sigma ref with
    | None -> NotAnElimination
    | Some c -> srec env 0 [] false c

let compute_consteval_mutual_fix env sigma ref =
  let rec srec env minarg labs ref c =
    let c',l = whd_betalet_stack sigma c in
    let nargs = List.length l in
    match EConstr.kind sigma c' with
      | Lambda (na,t,g) when List.is_empty l ->
          let open Context.Rel.Declaration in
	  srec (push_rel (LocalAssum (na,t)) env) (minarg+1) (t::labs) ref g
      | Fix ((lv,i),(names,_,_)) ->
	  (* Last known constant wrapping Fix is ref = [labs](Fix l) *)
	  (match compute_consteval_direct env sigma ref with
	     | NotAnElimination -> (*Above const was eliminable but this not!*)
		 NotAnElimination
	     | EliminationFix (minarg',minfxargs,infos) ->
		 let refs =
		   Array.map
		     (invert_name labs l names.(i) env sigma ref) names in
		 let new_minarg = max (minarg'+minarg-nargs) minarg' in
		 EliminationMutualFix (new_minarg,ref,(refs,infos))
	     | _ -> assert false)
      | _ when isEvalRef env sigma c' ->
	  (* Forget all \'s and args and do as if we had started with c' *)
	  let ref,_ = destEvalRefU sigma c' in
	  (match unsafe_reference_opt_value env sigma ref with
	    | None -> anomaly (Pp.str "Should have been trapped by compute_direct.")
	    | Some c -> srec env (minarg-nargs) [] ref c)
      | _ -> (* Should not occur *) NotAnElimination
  in
  match unsafe_reference_opt_value env sigma ref with
    | None -> (* Should not occur *) NotAnElimination
    | Some c -> srec env 0 [] ref c

let compute_consteval env sigma ref =
  match compute_consteval_direct env sigma ref with
    | EliminationFix (_,_,(nbfix,_,_)) when not (Int.equal nbfix 1) ->
	compute_consteval_mutual_fix env sigma ref
    | elim -> elim

let reference_eval env sigma = function
  | EvalConst cst as ref ->
      (try
	 Cmap.find cst !eval_table
       with Not_found -> begin
	 let v = compute_consteval env sigma ref in
	 eval_table := Cmap.add cst v !eval_table;
	 v
       end)
  | ref -> compute_consteval env sigma ref

(* If f is bound to EliminationFix (n',infos), then n' is the minimal
   number of args for starting the reduction and infos is
   (nbfix,[(yi1,Ti1);...;(yip,Tip)],n) indicating that f converts
   to some [y1:T1,...,yn:Tn](Fix(..) yip .. yi1) where the y_{i_j} consist in a
   disjoint subset of the yi, i.e. 1 <= ij <= n and the ij are disjoint (in
   particular, p <= n).

   f is applied to largs := arg1 .. argn and we need for recursive
   calls to build the function

      g := [xp:Tip',...,x1:Ti1'](f a1 ... an)

   s.t. (g u1 ... up) reduces to (Fix(..) u1 ... up)

   This is made possible by setting
      a_k:=x_j    if k=i_j for some j
      a_k:=arg_k  otherwise

   The type Tij' is Tij[yi(j-1)..y1 <- ai(j-1)..a1]
*)

let x = Name default_dependent_ident

let make_elim_fun (names,(nbfix,lv,n)) u largs =
  let lu = List.firstn n largs in
  let p = List.length lv in
  let lyi = List.map fst lv in
  let la =
    List.map_i (fun q aq ->
      (* k from the comment is q+1 *)
      try mkRel (p+1-(List.index Int.equal (n-q) lyi))
      with Not_found -> aq)
      0 (List.map (Vars.lift p) lu)
  in
  fun i ->
    match names.(i) with
      | None -> None
      | Some (minargs,ref) ->
          let body = applist (mkEvalRef ref u, la) in
          let g =
            List.fold_left_i (fun q (* j = n+1-q *) c (ij,tij) ->
              let subst = List.map (Vars.lift (-q)) (List.firstn (n-ij) la) in
              let tij' = Vars.substl (List.rev subst) tij in
	      mkLambda (x,tij',c)) 1 body (List.rev lv)
          in Some (minargs,g)

(* [f] is convertible to [Fix(recindices,bodynum),bodyvect)]:
   do so that the reduction uses this extra information *)

let dummy = mkProp
let vfx = Id.of_string "_expanded_fix_"
let vfun = Id.of_string "_eliminator_function_"
let venv = let open Context.Named.Declaration in
           val_of_named_context [LocalAssum (vfx, dummy); LocalAssum (vfun, dummy)]

(* Mark every occurrence of substituted vars (associated to a function)
   as a problem variable: an evar that can be instantiated either by
   vfx (expanded fixpoint) or vfun (named function). *)
let substl_with_function subst sigma constr =
  let evd = ref sigma in
  let minargs = ref Evar.Map.empty in
  let v = Array.of_list subst in
  let rec subst_total k c = match EConstr.kind sigma c with
  | Rel i when k < i ->
    if i <= k + Array.length v then
      match v.(i-k-1) with
      | (fx, Some (min, ref)) ->
        let sigma = !evd in
        let (sigma, evk) = Evarutil.new_pure_evar venv sigma dummy in
        evd := sigma;
        minargs := Evar.Map.add evk min !minargs;
        Vars.lift k (mkEvar (evk, [|fx;ref|]))
      | (fx, None) -> Vars.lift k fx
    else mkRel (i - Array.length v)
  | _ ->
    map_with_binders sigma succ subst_total k c in
  let c = subst_total 0 constr in
  (c, !evd, !minargs)

exception Partial

(* each problem variable that cannot be made totally applied even by
   reduction is solved by the expanded fix term. *)
let solve_arity_problem env sigma fxminargs c =
  let evm = ref sigma in
  let set_fix i = evm := Evd.define i (Constr.mkVar vfx) !evm in
  let rec check strict c =
    let c' = whd_betaiotazeta sigma c in
    let (h,rcargs) = decompose_app_vect sigma c' in
    match EConstr.kind sigma h with
        Evar(i,_) when Evar.Map.mem i fxminargs && not (Evd.is_defined !evm i) ->
          let minargs = Evar.Map.find i fxminargs in
          if Array.length rcargs < minargs then
            if strict then set_fix i
            else raise Partial;
          Array.iter (check strict) rcargs
      | (Var _|Const _) when isEvalRef env sigma h ->
          (let ev, u = destEvalRefU sigma h in
	     match reference_opt_value env sigma ev u with
             | Some h' ->
                let bak = !evm in
                (try Array.iter (check false) rcargs
                with Partial ->
                  evm := bak;
                  check strict (mkApp(h',rcargs)))
            | None -> Array.iter (check strict) rcargs)
      | _ -> EConstr.iter sigma (check strict) c' in
  check true c;
  !evm

let substl_checking_arity env subst sigma c =
  (* we initialize the problem: *)
  let body,sigma,minargs = substl_with_function subst sigma c in
  (* we collect arity constraints *)
  let sigma' = solve_arity_problem env sigma minargs body in
  (* we propagate the constraints: solved problems are substituted;
     the other ones are replaced by the function symbol *)
  let rec nf_fix c = match EConstr.kind sigma c with
  | Evar (i,[|fx;f|]) when Evar.Map.mem i minargs ->
    (** FIXME: find a less hackish way of doing this *)
    begin match EConstr.kind sigma' c with
    | Evar _ -> f
    | c -> EConstr.of_kind c
    end
  | _ -> EConstr.map sigma nf_fix c
  in
  nf_fix body

type fix_reduction_result = NotReducible | Reduced of (constr * constr list)

let reduce_fix whdfun sigma fix stack =
  match fix_recarg fix (Stack.append_app_list stack Stack.empty) with
    | None -> NotReducible
    | Some (recargnum,recarg) ->
        let (recarg'hd,_ as recarg') = whdfun sigma recarg in
        let stack' = List.assign stack recargnum (applist recarg') in
	(match EConstr.kind sigma recarg'hd with
           | Construct _ -> Reduced (contract_fix sigma fix, stack')
	   | _ -> NotReducible)

let contract_fix_use_function env sigma f
  ((recindices,bodynum),(_names,_types,bodies as typedbodies)) =
  let nbodies = Array.length recindices in
  let make_Fi j = (mkFix((recindices,j),typedbodies), f j) in
  let lbodies = List.init nbodies make_Fi in
  substl_checking_arity env (List.rev lbodies) sigma (nf_beta sigma bodies.(bodynum))

let reduce_fix_use_function env sigma f whfun fix stack =
  match fix_recarg fix (Stack.append_app_list stack Stack.empty) with
    | None -> NotReducible
    | Some (recargnum,recarg) ->
        let (recarg'hd,_ as recarg') =
	  if EConstr.isRel sigma recarg then
	    (* The recarg cannot be a local def, no worry about the right env *)
	    (recarg, [])
	  else
	    whfun recarg in
        let stack' = List.assign stack recargnum (applist recarg') in
	(match EConstr.kind sigma recarg'hd with
           | Construct _ ->
	       Reduced (contract_fix_use_function env sigma f fix,stack')
	   | _ -> NotReducible)

let contract_cofix_use_function env sigma f
  (bodynum,(_names,_,bodies as typedbodies)) =
  let nbodies = Array.length bodies in
  let make_Fi j = (mkCoFix(j,typedbodies), f j) in
  let subbodies = List.init nbodies make_Fi in
  substl_checking_arity env (List.rev subbodies)
    sigma (nf_beta sigma bodies.(bodynum))

let reduce_mind_case_use_function func env sigma mia =
  match EConstr.kind sigma mia.mconstr with
    | Construct ((ind_sp,i),u) ->
	let real_cargs = List.skipn mia.mci.ci_npar mia.mcargs in
	applist (mia.mlf.(i-1), real_cargs)
    | CoFix (bodynum,(names,_,_) as cofix) ->
	let build_cofix_name =
	  if isConst sigma func then
            let minargs = List.length mia.mcargs in
	    fun i ->
	      if Int.equal i bodynum then Some (minargs,func)
	      else match names.(i) with
		| Anonymous -> None
		| Name id ->
		    (* In case of a call to another component of a block of
		       mutual inductive, try to reuse the global name if
		       the block was indeed initially built as a global
		       definition *)
                    let (kn, u) = destConst sigma func in
                    let kn = con_with_label kn (Label.of_id id) in
                    let cst = (kn, EInstance.kind sigma u) in
		    try match constant_opt_value_in env cst with
		      | None -> None
                          (* TODO: check kn is correct *)
		      | Some _ -> Some (minargs,mkConstU (kn, u))
		    with Not_found -> None
	  else
	    fun _ -> None in
	let cofix_def =
          contract_cofix_use_function env sigma build_cofix_name cofix in
	mkCase (mia.mci, mia.mP, applist(cofix_def,mia.mcargs), mia.mlf)
    | _ -> assert false


let match_eval_ref env sigma constr stack =
  match EConstr.kind sigma constr with
  | Const (sp, u) ->
     reduction_effect_hook env sigma (EConstr.to_constr sigma constr)
        (lazy (EConstr.to_constr sigma (applist (constr,stack))));
     if is_evaluable env (EvalConstRef sp) then Some (EvalConst sp, u) else None
  | Var id when is_evaluable env (EvalVarRef id) -> Some (EvalVar id, EInstance.empty)
  | Rel i -> Some (EvalRel i, EInstance.empty)
  | Evar ev -> Some (EvalEvar ev, EInstance.empty)
  | _ -> None

let match_eval_ref_value env sigma constr stack =
  match EConstr.kind sigma constr with
  | Const (sp, u) ->
     reduction_effect_hook env sigma (EConstr.to_constr sigma constr)
        (lazy (EConstr.to_constr sigma (applist (constr,stack))));
    if is_evaluable env (EvalConstRef sp) then
      let u = EInstance.kind sigma u in
      Some (EConstr.of_constr (constant_value_in env (sp, u)))
    else
      None
  | Proj (p, c) when not (Projection.unfolded p) ->
     reduction_effect_hook env sigma (EConstr.to_constr sigma constr)
        (lazy (EConstr.to_constr sigma (applist (constr,stack))));
     if is_evaluable env (EvalConstRef (Projection.constant p)) then
       Some (mkProj (Projection.unfold p, c))
     else None
  | Var id when is_evaluable env (EvalVarRef id) -> 
     env |> lookup_named id |> NamedDecl.get_value
  | Rel n ->
     env |> lookup_rel n |> RelDecl.get_value |> Option.map (lift n)
  | _ -> None

let special_red_case env sigma whfun (ci, p, c, lf) =
  let rec redrec s =
    let (constr, cargs) = whfun s in
    match match_eval_ref env sigma constr cargs with
    | Some (ref, u) ->
      (match reference_opt_value env sigma ref u with
      | None -> raise Redelimination
      | Some gvalue ->
        if reducible_mind_case sigma gvalue then
	  reduce_mind_case_use_function constr env sigma
	  {mP=p; mconstr=gvalue; mcargs=cargs;
           mci=ci; mlf=lf}
	else
	  redrec (applist(gvalue, cargs)))
    | None ->
      if reducible_mind_case sigma constr then
        reduce_mind_case sigma
	  {mP=p; mconstr=constr; mcargs=cargs;
	  mci=ci; mlf=lf}
      else
	raise Redelimination
  in
  redrec c

let recargs = function
  | EvalVar _ | EvalRel _ | EvalEvar _ -> None
  | EvalConst c -> ReductionBehaviour.get (ConstRef c)

let reduce_projection env sigma pb (recarg'hd,stack') stack =
  (match EConstr.kind sigma recarg'hd with
  | Construct _ -> 
    let proj_narg = 
      pb.Declarations.proj_npars + pb.Declarations.proj_arg
    in Reduced (List.nth stack' proj_narg, stack)
  | _ -> NotReducible)

let reduce_proj env sigma whfun whfun' c =
  let rec redrec s =
    match EConstr.kind sigma s with
    | Proj (proj, c) -> 
      let c' = try redrec c with Redelimination -> c in
      let constr, cargs = whfun c' in
	(match EConstr.kind sigma constr with
	| Construct _ -> 
	  let proj_narg = 
	    let pb = lookup_projection proj env in
	      pb.Declarations.proj_npars + pb.Declarations.proj_arg
	  in List.nth cargs proj_narg
	| _ -> raise Redelimination)
    | Case (n,p,c,brs) -> 
      let c' = redrec c in
      let p = (n,p,c',brs) in
	(try special_red_case env sigma whfun' p
	 with Redelimination -> mkCase p)
    | _ -> raise Redelimination
  in redrec c

let whd_nothing_for_iota env sigma s =
  let rec whrec (x, stack as s) =
    match EConstr.kind sigma x with
      | Rel n ->
          let open Context.Rel.Declaration in
	  (match lookup_rel n env with
	     | LocalDef (_,body,_) -> whrec (lift n body, stack)
	     | _ -> s)
      | Var id ->
          let open Context.Named.Declaration in
	  (match lookup_named id env with
	     | LocalDef (_,body,_) -> whrec (body, stack)
	     | _ -> s)
      | Evar ev -> s
      | Meta ev ->
	(try whrec (EConstr.of_constr (Evd.meta_value sigma ev), stack)
	with Not_found -> s)
      | Const (const, u) when is_transparent_constant full_transparent_state const ->
          let u = EInstance.kind sigma u in
	  (match constant_opt_value_in env (const, u) with
	     | Some  body -> whrec (EConstr.of_constr body, stack)
	     | None -> s)
      | LetIn (_,b,_,c) -> stacklam whrec [b] sigma c stack
      | Cast (c,_,_) -> whrec (c, stack)
      | App (f,cl)  -> whrec (f, Stack.append_app cl stack)
      | Lambda (na,t,c) ->
          (match Stack.decomp stack with
             | Some (a,m) -> stacklam whrec [a] sigma c m
	     | _ -> s)

      | x -> s
  in
  EConstr.decompose_app sigma (Stack.zip sigma (whrec (s,Stack.empty)))

(* [red_elim_const] contracts iota/fix/cofix redexes hidden behind
   constants by keeping the name of the constants in the recursive calls;
   it fails if no redex is around *)

let rec red_elim_const env sigma ref u largs =
  let nargs = List.length largs in
  let largs, unfold_anyway, unfold_nonelim, nocase =
    match recargs ref with
    | None -> largs, false, false, false
    | Some (_,n,f) when nargs < n || List.mem `ReductionNeverUnfold f -> raise Redelimination
    | Some (x::l,_,_) when nargs <= List.fold_left max x l -> raise Redelimination
    | Some (l,n,f) ->
        let is_empty = match l with [] -> true | _ -> false in
	  reduce_params env sigma largs l, 
	  n >= 0 && is_empty && nargs >= n,
          n >= 0 && not is_empty && nargs >= n,
	  List.mem `ReductionDontExposeCase f
  in
  try match reference_eval env sigma ref with
    | EliminationCases n when nargs >= n ->
	let c = reference_value env sigma ref u in
	let c', lrest = whd_nothing_for_iota env sigma (applist(c,largs)) in
	let whfun = whd_simpl_stack env sigma in
        (special_red_case env sigma whfun (EConstr.destCase sigma c'), lrest), nocase
    | EliminationProj n when nargs >= n ->
	let c = reference_value env sigma ref u in
	let c', lrest = whd_nothing_for_iota env sigma (applist(c,largs)) in
	let whfun = whd_construct_stack env sigma in
	let whfun' = whd_simpl_stack env sigma in
	  (reduce_proj env sigma whfun whfun' c', lrest), nocase
    | EliminationFix (min,minfxargs,infos) when nargs >= min ->
	let c = reference_value env sigma ref u in
	let d, lrest = whd_nothing_for_iota env sigma (applist(c,largs)) in
	let f = make_elim_fun ([|Some (minfxargs,ref)|],infos) u largs in
	let whfun = whd_construct_stack env sigma in
	(match reduce_fix_use_function env sigma f whfun (destFix sigma d) lrest with
	   | NotReducible -> raise Redelimination
           | Reduced (c,rest) -> (nf_beta sigma c, rest), nocase)
    | EliminationMutualFix (min,refgoal,refinfos) when nargs >= min ->
	let rec descend (ref,u) args =
	  let c = reference_value env sigma ref u in
	  if evaluable_reference_eq sigma ref refgoal then
	    (c,args)
	  else
	    let c', lrest = whd_betalet_stack sigma (applist(c,args)) in
	    descend (destEvalRefU sigma c') lrest in
	let (_, midargs as s) = descend (ref,u) largs in
	let d, lrest = whd_nothing_for_iota env sigma (applist s) in
	let f = make_elim_fun refinfos u midargs in
	let whfun = whd_construct_stack env sigma in
	(match reduce_fix_use_function env sigma f whfun (destFix sigma d) lrest with
	   | NotReducible -> raise Redelimination
	   | Reduced (c,rest) -> (nf_beta sigma c, rest), nocase)
    | NotAnElimination when unfold_nonelim ->
         let c = reference_value env sigma ref u in
           (whd_betaiotazeta sigma (applist (c, largs)), []), nocase
    | _ -> raise Redelimination
    with Redelimination when unfold_anyway ->
       let c = reference_value env sigma ref u in
	 (whd_betaiotazeta sigma (applist (c, largs)), []), nocase

and reduce_params env sigma stack l =
  let len = List.length stack in
    List.fold_left (fun stack i ->
      if len <= i then raise Redelimination
      else
	let arg = List.nth stack i in
	let rarg = whd_construct_stack env sigma arg in
	  match EConstr.kind sigma (fst rarg) with
	  | Construct _ -> List.assign stack i (applist rarg)
	  | _ -> raise Redelimination)
      stack l
    

(* reduce to whd normal form or to an applied constant that does not hide
   a reducible iota/fix/cofix redex (the "simpl" tactic) *)

and whd_simpl_stack env sigma =
  let rec redrec s =
    let (x, stack) = decompose_app_vect sigma s in
    let stack = Array.to_list stack in
    let s' = (x, stack) in
    match EConstr.kind sigma x with
      | Lambda (na,t,c) ->
          (match stack with
             | [] -> s'
             | a :: rest -> redrec (beta_applist sigma (x, stack)))
      | LetIn (n,b,t,c) -> redrec (applist (Vars.substl [b] c, stack))
      | App (f,cl) -> redrec (applist(f, (Array.to_list cl)@stack))
      | Cast (c,_,_) -> redrec (applist(c, stack))
      | Case (ci,p,c,lf) ->
          (try
	    redrec (applist(special_red_case env sigma redrec (ci,p,c,lf), stack))
	  with
	      Redelimination -> s')
      | Fix fix ->
	  (try match reduce_fix (whd_construct_stack env) sigma fix stack with
            | Reduced s' -> redrec (applist s')
	    | NotReducible -> s'
	  with Redelimination -> s')

      | Proj (p, c) ->
        (try 
	   let unf = Projection.unfolded p in
	     if unf || is_evaluable env (EvalConstRef (Projection.constant p)) then
	       let pb = lookup_projection p env in
 		 (match unf, ReductionBehaviour.get (ConstRef (Projection.constant p)) with
 		 | false, Some (l, n, f) when List.mem `ReductionNeverUnfold f -> 
                   (* simpl never *) s'
		 | false, Some (l, n, f) when not (List.is_empty l) ->
		   let l' = List.map_filter (fun i -> 
		     let idx = (i - (pb.Declarations.proj_npars + 1)) in
		       if idx < 0 then None else Some idx) l in
		   let stack = reduce_params env sigma stack l' in
		     (match reduce_projection env sigma pb 
		       (whd_construct_stack env sigma c) stack 
		      with
		      | Reduced s' -> redrec (applist s')
		      | NotReducible -> s')
 		 | _ ->
		   match reduce_projection env sigma pb (whd_construct_stack env sigma c) stack with
		   | Reduced s' -> redrec (applist s')
		   | NotReducible -> s')
	   else s'
	 with Redelimination -> s')
	  
      | _ -> 
        match match_eval_ref env sigma x stack with
	| Some (ref, u) ->
          (try
	     let sapp, nocase = red_elim_const env sigma ref u stack in
             let hd, _ as s'' = redrec (applist(sapp)) in
             let rec is_case x = match EConstr.kind sigma x with
               | Lambda (_,_, x) | LetIn (_,_,_, x) | Cast (x, _,_) -> is_case x
               | App (hd, _) -> is_case hd
               | Case _ -> true
               | _ -> false in
               if nocase && is_case hd then raise Redelimination
               else s''
           with Redelimination -> s')
	| None -> s'
  in
  redrec

(* reduce until finding an applied constructor or fail *)

and whd_construct_stack env sigma s =
  let (constr, cargs as s') = whd_simpl_stack env sigma s in
  if reducible_mind_case sigma constr then s'
  else match match_eval_ref env sigma constr cargs with
  | Some (ref, u) ->
    (match reference_opt_value env sigma ref u with
    | None -> raise Redelimination
    | Some gvalue -> whd_construct_stack env sigma (applist(gvalue, cargs)))
  | _ -> raise Redelimination

(************************************************************************)
(*            Special Purpose Reduction Strategies                     *)

(* Red reduction tactic: one step of delta reduction + full
   beta-iota-fix-cofix-zeta-cast at the head of the conclusion of a
   sequence of products; fails if no delta redex is around
*)

let try_red_product env sigma c =
  let simpfun c = clos_norm_flags betaiotazeta env sigma c in
  let rec redrec env x =
    let x = whd_betaiota sigma x in
    match EConstr.kind sigma x with
      | App (f,l) ->
          (match EConstr.kind sigma f with
             | Fix fix ->
                 let stack = Stack.append_app l Stack.empty in
                 (match fix_recarg fix stack with
                    | None -> raise Redelimination
                    | Some (recargnum,recarg) ->
                        let recarg' = redrec env recarg in
                        let stack' = Stack.assign stack recargnum recarg' in
                        simpfun (Stack.zip sigma (f,stack')))
             | _ -> simpfun (mkApp (redrec env f, l)))
      | Cast (c,_,_) -> redrec env c
      | Prod (x,a,b) ->
          let open Context.Rel.Declaration in
	  mkProd (x, a, redrec (push_rel (LocalAssum (x, a)) env) b)
      | LetIn (x,a,b,t) -> redrec env (Vars.subst1 a t)
      | Case (ci,p,d,lf) -> simpfun (mkCase (ci,p,redrec env d,lf))
      | Proj (p, c) -> 
	let c' = 
	  match EConstr.kind sigma c with
	  | Construct _ -> c
	  | _ -> redrec env c
	in
	let pb = lookup_projection p env in
          (match reduce_projection env sigma pb (whd_betaiotazeta_stack sigma c') [] with
	  | Reduced s -> simpfun (applist s)
	  | NotReducible -> raise Redelimination)
      | _ -> 
        (match match_eval_ref env sigma x [] with
        | Some (ref, u) ->
          (* TO DO: re-fold fixpoints after expansion *)
          (* to get true one-step reductions *)
	  (match reference_opt_value env sigma ref u with
	     | None -> raise Redelimination
	     | Some c -> c)
	| _ -> raise Redelimination)
  in redrec env c

let red_product env sigma c =
  try try_red_product env sigma c
  with Redelimination -> user_err (str "No head constant to reduce.")

(*
(* This old version of hnf uses betadeltaiota instead of itself (resp
   whd_construct_state) to reduce the argument of Case (resp Fix);
   The new version uses the "simpl" strategy instead. For instance,

   Variable n:nat.
   Eval hnf in match (plus (S n) O) with S n => n | _ => O end.

   returned

   (fix plus (n m : nat) {struct n} : nat :=
        match n with
        | O => m
        | S p => S (plus p m)
        end) n 0

   while the new version returns (plus n O)
 *)

let whd_simpl_orelse_delta_but_fix_old env sigma c =
  let whd_all = whd_all_state env sigma in
  let rec redrec (x, stack as s) =
    match kind_of_term x with
      | Lambda (na,t,c) ->
          (match decomp_stack stack with
             | None      -> s
             | Some (a,rest) -> stacklam redrec [a] c rest)
      | LetIn (n,b,t,c) -> stacklam redrec [b] c stack
      | App (f,cl)   -> redrec (f, append_stack cl stack)
      | Cast (c,_,_) -> redrec (c, stack)
      | Case (ci,p,d,lf) ->
          (try
             redrec (special_red_case env sigma whd_all (ci,p,d,lf), stack)
           with Redelimination ->
	     s)
      | Fix fix ->
	  (match reduce_fix whd_all fix stack with
             | Reduced s' -> redrec s'
	     | NotReducible -> s)
      | _ when isEvalRef env x ->
	  let ref = destEvalRef x in
          (try
	    redrec (red_elim_const env sigma ref stack)
           with Redelimination ->
             match reference_opt_value env sigma ref with
	       | Some c ->
		   (match kind_of_term (strip_lam c) with
                     | CoFix _ | Fix _ -> s
		     | _ -> redrec (c, stack))
	       | None -> s)
      | _ -> s
  in app_stack (redrec (c, empty_stack))
*)

let whd_simpl_stack = 
  if Flags.profile then 
    let key = Profile.declare_profile "whd_simpl_stack" in
      Profile.profile3 key whd_simpl_stack
  else whd_simpl_stack

(* Same as [whd_simpl] but also reduces constants that do not hide a
   reducible fix, but does this reduction of constants only until it
   immediately hides a non reducible fix or a cofix *)

let whd_simpl_orelse_delta_but_fix env sigma c =
  let rec redrec s =
    let (constr, stack as s') = whd_simpl_stack env sigma s in
    match match_eval_ref_value env sigma constr stack with
    | Some c ->
      (match EConstr.kind sigma (snd (decompose_lam sigma c)) with
      | CoFix _ | Fix _ -> s'
      | Proj (p,t) when
	  (match EConstr.kind sigma constr with
	  | Const (c', _) -> eq_constant (Projection.constant p) c'
	  | _ -> false) ->
	let pb = Environ.lookup_projection p env in
	  if List.length stack <= pb.Declarations.proj_npars then
	    (** Do not show the eta-expanded form *)
	    s'
	  else redrec (applist (c, stack))
      | _ -> redrec (applist(c, stack)))
    | None -> s'
  in
  let simpfun = clos_norm_flags betaiota env sigma in
  simpfun (applist (redrec c))

let hnf_constr = whd_simpl_orelse_delta_but_fix

(* The "simpl" reduction tactic *)

let whd_simpl env sigma c =
  applist (whd_simpl_stack env sigma c)

let simpl env sigma c = strong whd_simpl env sigma c

(* Reduction at specific subterms *)

let matches_head env sigma c t =
  match EConstr.kind sigma t with
    | App (f,_) -> Constr_matching.matches env sigma c f
    | Proj (p, _) -> Constr_matching.matches env sigma c (mkConstU (Projection.constant p, EInstance.empty))
    | _ -> raise Constr_matching.PatternMatchingFailure

(** FIXME: Specific function to handle projections: it ignores what happens on the
    parameters. This is a temporary fix while rewrite etc... are not up to equivalence
    of the projection and its eta expanded form.
*)
let change_map_constr_with_binders_left_to_right g f (env, l as acc) sigma c = 
  match EConstr.kind sigma c with
  | Proj (p, r) -> (* Treat specially for partial applications *)
    let t = Retyping.expand_projection env sigma p r [] in
    let hdf, al = destApp sigma t in
    let a = al.(Array.length al - 1) in
    let app = (mkApp (hdf, Array.sub al 0 (Array.length al - 1))) in
    let app' = f acc app in
    let a' = f acc a in
      (match EConstr.kind sigma app' with
      | App (hdf', al') when hdf' == hdf ->
        (* Still the same projection, we ignore the change in parameters *)
	mkProj (p, a')
      | _ -> mkApp (app', [| a' |]))
  | _ -> map_constr_with_binders_left_to_right sigma g f acc c

let e_contextually byhead (occs,c) f = begin fun env sigma t ->
  let (nowhere_except_in,locs) = Locusops.convert_occs occs in
  let maxocc = List.fold_right max locs 0 in
  let pos = ref 1 in
  (** FIXME: we do suspicious things with this evarmap *)
  let evd = ref sigma in
  let rec traverse nested (env,c as envc) t =
    if nowhere_except_in && (!pos > maxocc) then (* Shortcut *) t
    else
    try
      let subst =
        if byhead then matches_head env sigma c t 
	else Constr_matching.matches env sigma c t in
      let ok =
	if nowhere_except_in then Int.List.mem !pos locs
	else not (Int.List.mem !pos locs) in
      incr pos;
      if ok then begin
        if Option.has_some nested then
          user_err  (str "The subterm at occurrence " ++ int (Option.get nested) ++ str " overlaps with the subterm at occurrence " ++ int (!pos-1) ++ str ".");
        (* Skip inner occurrences for stable counting of occurrences *)
        if locs != [] then
          ignore (traverse_below (Some (!pos-1)) envc t);
	let (evm, t) = (f subst) env !evd t in
	(evd := evm; t)
      end
      else
	traverse_below nested envc t
    with Constr_matching.PatternMatchingFailure ->
      traverse_below nested envc t
  and traverse_below nested envc t =
    (* when byhead, find other occurrences without matching again partial
       application with same head *)
    match EConstr.kind !evd t with
    | App (f,l) when byhead -> mkApp (f, Array.map_left (traverse nested envc) l)
    | Proj (p,c) when byhead -> mkProj (p,traverse nested envc c)
    | _ ->
        change_map_constr_with_binders_left_to_right
          (fun d (env,c) -> (push_rel d env,lift_pattern 1 c))
          (traverse nested) envc sigma t
  in
  let t' = traverse None (env,c) t in
  if List.exists (fun o -> o >= !pos) locs then error_invalid_occurrence locs;
  (!evd, t')
  end

let contextually byhead occs f env sigma t =
  let f' subst env sigma t = sigma, f subst env sigma t in
  snd (e_contextually byhead occs f' env sigma t)

(* linear bindings (following pretty-printer) of the value of name in c.
 * n is the number of the next occurrence of name.
 * ol is the occurrence list to find. *)

let match_constr_evaluable_ref sigma c evref = 
  match EConstr.kind sigma c, evref with
  | Const (c,u), EvalConstRef c' when eq_constant c c' -> Some u
  | Var id, EvalVarRef id' when id_eq id id' -> Some EInstance.empty
  | _, _ -> None

let substlin env sigma evalref n (nowhere_except_in,locs) c =
  let maxocc = List.fold_right max locs 0 in
  let pos = ref n in
  assert (List.for_all (fun x -> x >= 0) locs);
  let value u = value_of_evaluable_ref env evalref u in
  let rec substrec () c =
    if nowhere_except_in && !pos > maxocc then c
    else 
      match match_constr_evaluable_ref sigma c evalref with
      | Some u ->
        let ok =
	  if nowhere_except_in then Int.List.mem !pos locs
	  else not (Int.List.mem !pos locs) in
	  incr pos;
	  if ok then value u else c
      | None -> 
        map_constr_with_binders_left_to_right sigma
	  (fun _ () -> ())
          substrec () c
  in
  let t' = substrec () c in
  (!pos, t')

let string_of_evaluable_ref env = function
  | EvalVarRef id -> Id.to_string id
  | EvalConstRef kn ->
      string_of_qualid
        (Nametab.shortest_qualid_of_global (vars_of_env env) (ConstRef kn))

let unfold env sigma name c =
  if is_evaluable env name then
    clos_norm_flags (unfold_red name) env sigma c
  else
    user_err Pp.(str (string_of_evaluable_ref env name^" is opaque."))

(* [unfoldoccs : (readable_constraints -> (int list * full_path) -> constr -> constr)]
 * Unfolds the constant name in a term c following a list of occurrences occl.
 * at the occurrences of occ_list. If occ_list is empty, unfold all occurrences.
 * Performs a betaiota reduction after unfolding. *)
let unfoldoccs env sigma (occs,name) c =
  let unfo nowhere_except_in locs =
    let (nbocc,uc) = substlin env sigma name 1 (nowhere_except_in,locs) c in
    if Int.equal nbocc 1 then
      user_err Pp.(str ((string_of_evaluable_ref env name)^" does not occur."));
    let rest = List.filter (fun o -> o >= nbocc) locs in
    let () = match rest with
    | [] -> ()
    | _ -> error_invalid_occurrence rest
    in
    nf_betaiotazeta sigma uc
  in
  match occs with
    | NoOccurrences -> c
    | AllOccurrences -> unfold env sigma name c
    | OnlyOccurrences l -> unfo true l
    | AllOccurrencesBut l -> unfo false l

(* Unfold reduction tactic: *)
let unfoldn loccname env sigma c =
  List.fold_left (fun c occname -> unfoldoccs env sigma occname c) c loccname

(* Re-folding constants tactics: refold com in term c *)
let fold_one_com com env sigma c =
  let rcom =
    try red_product env sigma com
    with Redelimination -> user_err Pp.(str "Not reducible.") in
  (* Reason first on the beta-iota-zeta normal form of the constant as
     unfold produces it, so that the "unfold f; fold f" configuration works
     to refold fix expressions *)
  let a = subst_term sigma (clos_norm_flags unfold_side_red env sigma rcom) c in
  if not (EConstr.eq_constr sigma a c) then
    Vars.subst1 com a
  else
    (* Then reason on the non beta-iota-zeta form for compatibility -
       even if it is probably a useless configuration *)
    let a = subst_term sigma rcom c in
    Vars.subst1 com a

let fold_commands cl env sigma c =
  List.fold_right (fun com c -> fold_one_com com env sigma c) (List.rev cl) c


(* call by value reduction functions *)
let cbv_norm_flags flags env sigma t =
  cbv_norm (create_cbv_infos flags env sigma) t

let cbv_beta = cbv_norm_flags beta empty_env
let cbv_betaiota = cbv_norm_flags betaiota empty_env
let cbv_betadeltaiota env sigma =  cbv_norm_flags all env sigma

let compute = cbv_betadeltaiota

(* Pattern *)

(* gives [na:ta]c' such that c converts to ([na:ta]c' a), abstracting only
 * the specified occurrences. *)

let abstract_scheme env sigma (locc,a) (c, sigma) =
  let ta = Retyping.get_type_of env sigma a in
  let na = named_hd env sigma ta Anonymous in
  if occur_meta sigma ta then user_err Pp.(str "Cannot find a type for the generalisation.");
  if occur_meta sigma a then
    mkLambda (na,ta,c), sigma
  else
    let c', sigma' = subst_closed_term_occ env sigma (AtOccs locc) a c in
      mkLambda (na,ta,c'), sigma'

let pattern_occs loccs_trm = begin fun env sigma c ->
  let abstr_trm, sigma = List.fold_right (abstract_scheme env sigma) loccs_trm (c,sigma) in
  try
    let _ = Typing.unsafe_type_of env sigma abstr_trm in
    (sigma, applist(abstr_trm, List.map snd loccs_trm))
  with Type_errors.TypeError (env',t) ->
    raise (ReductionTacticError (InvalidAbstraction (env,sigma,abstr_trm,(env',t))))
  end

(* Used in several tactics. *)

let check_privacy env ind =
  let spec = Inductive.lookup_mind_specif env (fst ind) in
  if Inductive.is_private spec then
    user_err  (str "case analysis on a private type.")
  else ind

let check_not_primitive_record env ind =
  let spec = Inductive.lookup_mind_specif env (fst ind) in
    if Inductive.is_primitive_record spec then
      user_err  (str "case analysis on a primitive record type: " ++
		       str "use projections or let instead.")
    else ind

(* put t as t'=(x1:A1)..(xn:An)B with B an inductive definition of name name
   return name, B and t' *)

let reduce_to_ind_gen allow_product env sigma t =
  let rec elimrec env t l =
    let t = hnf_constr env sigma t in
    match EConstr.kind sigma (fst (decompose_app_vect sigma t)) with
      | Ind ind-> (check_privacy env ind, it_mkProd_or_LetIn t l)
      | Prod (n,ty,t') ->
	  let open Context.Rel.Declaration in
	  if allow_product then
	    elimrec (push_rel (LocalAssum (n,ty)) env) t' ((LocalAssum (n,ty))::l)
	  else
	    user_err  (str"Not an inductive definition.")
      | _ ->
	  (* Last chance: we allow to bypass the Opaque flag (as it
	     was partially the case between V5.10 and V8.1 *)
	  let t' = whd_all env sigma t in
	  match EConstr.kind sigma (fst (decompose_app_vect sigma t')) with
	    | Ind ind-> (check_privacy env ind, it_mkProd_or_LetIn t' l)
	    | _ -> user_err  (str"Not an inductive product.")
  in
  elimrec env t []

let reduce_to_quantified_ind env sigma c = reduce_to_ind_gen true env sigma c
let reduce_to_atomic_ind env sigma c = reduce_to_ind_gen false env sigma c

let find_hnf_rectype env sigma t =
  let ind,t = reduce_to_atomic_ind env sigma t in
  ind, snd (decompose_app sigma t)

(* Reduce the weak-head redex [beta,iota/fix/cofix[all],cast,zeta,simpl/delta]
   or raise [NotStepReducible] if not a weak-head redex *)

exception NotStepReducible

let one_step_reduce env sigma c =
  let rec redrec (x, stack) =
    match EConstr.kind sigma x with
      | Lambda (n,t,c)  ->
          (match stack with
             | []        -> raise NotStepReducible
             | a :: rest -> (Vars.subst1 a c, rest))
      | App (f,cl) -> redrec (f, (Array.to_list cl)@stack)
      | LetIn (_,f,_,cl) -> (Vars.subst1 f cl,stack)
      | Cast (c,_,_) -> redrec (c,stack)
      | Case (ci,p,c,lf) ->
          (try
	     (special_red_case env sigma (whd_simpl_stack env sigma)
	       (ci,p,c,lf), stack)
           with Redelimination -> raise NotStepReducible)
      | Fix fix ->
	  (try match reduce_fix (whd_construct_stack env) sigma fix stack with
             | Reduced s' -> s'
	     | NotReducible -> raise NotStepReducible
           with Redelimination -> raise NotStepReducible)
      | _ when isEvalRef env sigma x ->
	  let ref,u = destEvalRefU sigma x in
          (try
             fst (red_elim_const env sigma ref u stack)
           with Redelimination ->
	     match reference_opt_value env sigma ref u with
	       | Some d -> (d, stack)
	       | None -> raise NotStepReducible)

      | _ -> raise NotStepReducible
  in
  applist (redrec (c,[]))

let error_cannot_recognize ref =
  user_err 
    (str "Cannot recognize a statement based on " ++
     Nametab.pr_global_env Id.Set.empty ref ++ str".")

let reduce_to_ref_gen allow_product env sigma ref t =
  if isIndRef ref then
    let ((mind,u),t) = reduce_to_ind_gen allow_product env sigma t in
    begin match ref with
    | IndRef mind' when eq_ind mind mind' -> t
    | _ -> error_cannot_recognize ref
    end
  else
  (* lazily reduces to match the head of [t] with the expected [ref] *)
  let rec elimrec env t l =
    let c, _ = decompose_app_vect sigma t in
    match EConstr.kind sigma c with
      | Prod (n,ty,t') ->
          if allow_product then
	    let open Context.Rel.Declaration in
	    elimrec (push_rel (LocalAssum (n,t)) env) t' ((LocalAssum (n,ty))::l)
          else
            error_cannot_recognize ref
      | _ ->
	  try
	    if eq_gr (fst (global_of_constr sigma c)) ref
	    then it_mkProd_or_LetIn t l
	    else raise Not_found
	  with Not_found ->
          try
	    let t' = nf_betaiota sigma (one_step_reduce env sigma t) in
            elimrec env t' l
          with NotStepReducible -> error_cannot_recognize ref
  in
  elimrec env t []

let reduce_to_quantified_ref = reduce_to_ref_gen true
let reduce_to_atomic_ref = reduce_to_ref_gen false